People in Japan know that Americans do tip and they will not be so surprised. Specially if you look like a "gaijin" tourist.
Some will even consider it fun to be tipped for first time in their life, others will find it offensive and will think that you are looking down to them. But, in any case, tipping is a faux pas in Japan.
I live in Japan and my wife is Japanese. Also, I had this conversation with several Japanese people.
I can tell you that most Japanese I know hate the concept of tipping and it makes them uncomfortable. They also hate giving tips. Not because of the money, but because they think that tipping is a way of looking down the other person.
Of course, when I say offensive I don't mean that they will curse and hate you forever. But probably some will think: "I already have a salary, I don't need you give me your coins".
If there's an scale of socially awkward situations, I guess tipping would be slightly under asking ketchup for your sashimi. But definitely not as bad as going into someone's house with your shoes on.
I don't know. Before I was made aware of this, I tried tipping while in Japan. Several places people were very dismissive about it. Eventually I just stopped, because it seemed like people didn't want tips.
It's correct to say that in Japan one should not tip. It's not correct to say that it's considered offensive.
To seriously offend some Japanese people you'll have to do something crazy like stick your chopsticks straight up in your rice bowl. Then you'll see them really wince ;-)
Some will even consider it fun to be tipped for first time in their life, others will find it offensive and will think that you are looking down to them. But, in any case, tipping is a faux pas in Japan.